Thousands of people flocked to Khartoum airport to welcome 40 members of the Ummah party.

One of Sudan's vice-presidents greeted them on behalf of the government.

A spokesman for the exiles headed by the former prime minister, Sadiq al-Mahdi, said they had returned to strengthen democracy and work for peace.

Mr Mahdi, who was overthrown by President Omar Bashir in 1989, is not returning, but said his party wanted to try to exploit what it saw as a greater margin of freedom in Khartoum.

He also said there was less scope for the opposition abroad as Sudan improved ties with neighbours such as Egypt and Eritrea.

"There could be some restrictions, but we think there is enough freedom to move and we say that yes, there could be problems, but nevertheless, we reckon that it is time for us to go and activate our organisation inside Sudan," Mr Mahdi said.

Last month, the Ummah party withdrew from the opposition National Democratic Alliance, which groups northern and southern rebels, dominated by the Sudan People's Liberation Army (SPLA).

The Sudanese opposition is split over strategy, and there has been a recent upsurge in fighting in Sudan's long civil war.